Silicon microphones, or silicon based MEMS microphones, also known as acoustic transducers, have been in research and development for many years. The silicon microphones may be widely used in many applications, such as cell phones, tablet PCs, cameras, hearing aids, smart toys and surveillance devices due to their potential advantages in miniaturization, performances, reliability, environmental endurance, costs and mass production capability.
A typical silicon microphone comprises a highly flexible diaphragm stacked on a silicon substrate and exposed to the outside through a back hole formed in the silicon substrate, and a fixed perforated backplate located above the diaphragm with an air gap in between. The flexible diaphragm and the perforated backplate forms a variable air-gap condenser, which can convert acoustic energy into electric energy for detection when the diaphragm vibrates in response to an external acoustic wave or a sound pressure impact reaching the diaphragm through the back hole.
As a critical part of the silicon microphone, the diaphragm play a very important role in determining the performances of the microphone, for example, large tensile stress in the diaphragm can lead to undesirable effects such as low and irreproducible sensitivity of the microphone. Therefore, it is one of major subject matters for a microphone designer to reduce the stress in the diaphragm in order to improve the performances, such as sensitivity and reproducibility, of the microphone.
Patent application No. PCT/CN2013/080908 disclosed a silicon microphone with a stopper structure, in which a narrow slot is formed around the edge of the diaphragm to release the stress in the diaphragm, however, the slot makes the microphone fragile in a drop test, and the stopper structure introduced for improving drop performance nevertheless increases process complexity and thus yields a cost concern. Other stress-free diaphragm designs, such as the floating diaphragm scheme of Knowles Corporation, the folded-spring supported diaphragm scheme of Analog Devices, Inc. and so on, are also preferred for production control and for high reliability as well. However, they either involves complicated fabrication procedures, or becomes too fragile for a drop test
Q. Zou et al (Quanbo Zou, Zhijian Li, and Litian Liu, Design and Fabrication of Silicon Condenser Microphone Using Corrugated Diaphragm Technique, Journal of Microelectromechanical Systems, Vol. 5, No. 3, September 1996) proposed a single wafer condenser microphone with rectangular shaped corrugations formed all over the diaphragm, which is advantageous in reducing initial stress in the diaphragm and rendering the microphone a high sensitivity. However, the disadvantages of the microphone are that bridging parts are needed to be formed in the diaphragm for the backplate process and thus stress release for the diaphragm is not complete, and on the other hand, multiple corrugations on the whole diaphragm will actually increase the bending stiffness of the diaphragm and thus reduce the sensitivity of the microphone.
P. Scheeper et al (Patrick R. Scheeper, Wouter Olthuis, and Piet Bergveld, The Design, Fabrication and Testing of Corrugated Silicon Nitride Diaphragms, Journal of Microelectromechanical Systems, Vol. 3, No. 1, March 1994) disclosed a corrugated silicon nitride diaphragm for improving mechanical sensitivity under initial tensile stress. The mechanical sensitivity of the diaphragm is significantly improved, but at a penalty of increased diaphragm area (about 3 times larger than a normal design). A large static deflection of the diaphragm up to 2˜7 μm is observed due to a large diaphragm area having multiple (8) ring-shaped corrugations formed therein, which made it unrealistic to construct a MEMS microphone in which the air gap between the diaphragm and the backplate is typically a few microns. Besides, multiple corrugations on the diaphragm will also increase the bending stiffness of the diaphragm and thus reduce the sensitivity of the diaphragm.